Question 1052726
-36+39x-3x^3=f(x)

the derivative is -9x^2+39
Set it equal to 0 and solve
9x^2=39
x^2=39/9
x=+/-sqrt(39)/3
Numerically, this is +/-2.082
Graphically, the y value is -90.12 at x=-2.08 and 18.12 and x=+2.08
{{{graph(300,300,-10,10,-100,50,-3x^3+39x-36)}}}